In the powder coating of workpieces, especially coating of the latter with synthetic resin powders, such as in the synthetic resin powder coating of can bodies, inter alia their longitudinal weld seam, the problem arises basically that environmental factors, such as atmospheric humidity, degree of contamination, affect the properties of the film formed on the workpiece by repeated melting of the applied powder. Such coating procedures are frequently utilized in line in the immediate vicinity with other processing stations, such as with welding units for the longitudinal seams of can bodies as the workpieces; as a consequence, the ambient air is contaminated with oil vapors and other vaporization products of the welding step.
The invention has the objective of at least reducing effects of the environment on the result of the coating process; this objective is achieved in a process of the above kind in accordance with the invention by conditioning the air and at least extensively preventing penetration of ambient air at least to the coating zone for the workpiece.
On account of the feature that the air by means of which the powder is conveyed to the powder feeder is conditioned, i.e. its atmospheric humidity is maintained at a predetermined value, and that air is used, the residual contamination degree of which influences the powder coating only to a negligible extent, in conjunction with the feature that air from the immediate surroundings is precluded from penetrating to the coating zone, the objective is attained that the coating procedure can take place under predetermined, controlled environmental parameters. In this connection, first priority must be given to the essential feature that such measures are to be taken at the coating zone proper, i.e. at the location where the powder is delivered and applied to the workpiece. Additionally, an environment, controllable as described above, is maintained with respect to the workpiece with the powder application preferably for such a time period until the powder coating to be produced can no longer be impaired by fluctuating environmental influences.
In conventional powder coating processes wherein workpieces are coated in series one after the other, the powder feeder being moved relatively to the workpiece during the coating step, the additional suggestion is advanced, for preventing penetration of ambient air, to provide a chamber that is open at least on one side for the relative movement between workpiece and powder feeder, and to produce an air flow out of the chamber opening into the surroundings.
By providing such an air flow from the chamber arranged around the coating zone, it is ensured that only conditioned air, thus the powder conveying air, enters the chamber.
It is furthermore known in powder coating processes to return by suction any powder not applied to the workpiece and/or not remaining applied to the workpiece, this sucked-back powder usually being recycled to the powder feeder in a conveying cycle. The powder, delivered in this arrangement in a closed cycle repeatedly to the coating zone, which usually is in free communication with the immediate environment, becomes increasingly soiled at the coating zone and also absorbs increasing amounts of moisture. Even though the problem of growing moisture in the powder can perhaps be controlled in a powder processing unit at relatively high expense, it is impossible in the aforementioned conveying cycle to keep the repeatedly fed powder clean at reasonable expense. For this reason, quite special significance resides in utilizing the process according to this invention in a powder coating procedure wherein excess powder is repeatedly dispensed in a conveying cycle along the above lines. A powder reprocessing facility and/or stage, otherwise required, thus becomes superfluous.
In a coating process wherein at least one exhaust for air and powder not applied to the workpiece and possibly, as explained above, returned in a conveying cycle to the powder feeder, is provided, it is furthermore suggested that the air flow out of the chamber opening is produced by feeding, per unit time, a larger quantity of conditioned air to the chamber than the amount of air exhausted therefrom. In this connection, the aforementioned balance of conditioned air introduced into the chamber and air removed from the chamber can also be influenced in the aforementioned sense by generating the air flow by conditioned air fed into the chamber in addition to the conditioned conveying air.
If the aforementioned outward flow from the chamber is produced exclusively with conditioned air by designing the above-mentioned balance per unit time correspondingly large in favor of the conditioned air that is introduced, then a relatively large quantity of conditioned air is used up, entailing a corresponding design of a climatizing stage and perhaps purification stage for the aforementioned air. Also, it is to be kept in mind that the amount of conditioned air conveyed per unit time is not arbitrarily adjustable because this amount directly affects the powder coating process, for example via the delivery rate. For this reason, it is desirable in most cases to separate setting parameters for the coating step from setting parameters for the flow according to this invention. To this end, conditioned air without powder is introduced into the chamber in addition to the aforementioned conveying air.
The feature of passing air through nozzles furthermore in the region of the chamber opening, and in such a case preferably unconditioned air, so that a flow of conditioned air out of the opening of the chamber toward the outside is produced in the chamber by jet pump effect, provides the result that, on the one hand, less conditioned air needs to be utilized in that the pump jet forms practically a barrier against the influx of ambient air into the chamber.
In order to prevent excess powder from settling on the chamber wall, it is furthermore proposed to electrically charge the powder predominantly in one polarity, the powder being electrostatically repelled by the chamber. This is done preferably in case the powder coating step is effected with electrostatic enhancement by producing an electrostatic field in the zone of the workpiece, charging the delivered powder, and driving the latter by the force of the field against the workpiece. In this case, for example, a metallized inside wall of the aforementioned chamber, designed insulated toward the outside for protection against electric shock, is placed at the same electric potential as an electrode in the coating zone, this electrode generating, together with the workpiece placed at corresponding electric potential, the electrostatic field for applying the powder.
A powder coating facility according to this invention, where environmental influences are mitigated, comprises a conditioning chamber, at least one feed conduit extended into the chamber and terminating in a coating zone for powder conveyed by means of conditioned air, an air conditioning device connected to the feed conduit as well as likewise connected to conveying means for the conditioned air with powder, at least one opening in the chamber for the introduction of a workpiece to be coated, and means for producing at the chamber opening an air flow oriented toward the surroundings of the opening. The process of this invention as well as the powder coating facility according to this invention are suited, in particular, for the internal coating of hollow items, in this connection also especially for the seam coating of longitudinal weld seams in can bodies, taking place customarily directly downstream of a welding facility where the contaminations mentioned in the foregoing are relatively pronounced. The aforementioned process and, respectively, coating facility, however, can also be utilized for the all-around internal coatings of hollow articles.